Tuning indicator circuit



Oct. 5, 1937. F. H. SHEPARD. JR

TUNING INDICATOR CIRCUIT Filed Aug. 26, 1936 E f0 7] AAAAAAZAL 3 I. v Lv W7 2 v A 2 v 9 2 4 Z INVENTOR I FRANCIS H. SHEPARQJR.

ATTORNEY Patented Oct. 5, 1937 UNITED STATES TUNING INDICATOR CIRCUIT Francis H. Shepard, J12,

Rutherford, N. J assignor, by mesne assignments, to Radio Corporation of America, a corporation of Delaware Application August 26, 1936, Serial No. 98,005

8 Claims.

My invention relates to radio receiving circuits, and more particularly to such circuits incorporating visual tuning indicators which respond to voltage developed when the circuit is properly tuned.

Voltage or tuning indicators which show the presence of the voltage characteristic of a properly tuned receiving'circuit are in commercial use. Tuning indicators of the electron discharge 10 type usually have an element, such as an anode or plate, which is rendered luminous to a greater or less extent by an electron discharge influenced by a control electrode on which a voltage corresponding to the tuning of the receiver circuit is impressed. When the receiving circuit is tuned to resonance with an incoming signal the voltage developed and impressed on the control electrode of the tuning indicator enables the control electrode to control the electron discharge so as to make the luminous portion of the plate of a predetermined width. The device is usually so connected that the luminous portion of the anode of the tuning indicator portion is of maximum width and the dark portion is of minimum width, and preferably the entire anode is fluorescent and the dark portion absent when the circuit is tuned to resonance.

In a conventional radio receiving circuit provided with automatic volume control the. visual tuning indicator above described may be connected in the output of the rectifier which develops a direct voltage depending upon the strength of the signal for use in the automatic volume control circuit. In some of the simpler circuits where the number of tubes used is small the automatic volume control arrangement is only partially efiective to maintain the volume substantially constant on very strong signals or very weak signals. Inasmuch as the visual tuning indicator is connected to the rectifier which develops the voltage for the automatic volume control, it will also be effected by the inability of the circuit to maintain substantially constant conditions on very strong or very weak signals so that when the visual tuning indicator is connected to respond fully to proper tuning on weak signals the voltage impressed on the indicator by correct tuning on strong signals is so great that the luminescent pattern on the indicating portion of the'tube overlaps so that the indicator. is practically inoperative when strong signals are being received On the other hand if the visual indicator is connected to respond properly to tuning on strong signals then tuning on weak signals impresses such a slight voltage on the indicator that very little indication is shown by the luminous pattern on the target. This difliculty is also present with other types of Visual indicators which respond to voltage develop-ed when the receiving circuit is tuned.

The principal object of my invention is to provide an improved radio receiving circuit having a visual tuning indicatorwhich can be. adjusted to respond properly to tuning of the receiver circuit on both strong and weak signals.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure of the drawing is a diagrammatic representation of a radio receiving circuit embodying my invention.

In one preferred embodiment of my invention the radio receiving circuit may include a radio frequency amplifier upon which the signals are impressed and which is coupled to detector or rectifier from which the automatic volume control voltage is obtained. The output of the detector may be coupled in turn to an audio frequency amplifier through a volume control arrangement which determines the amplitude of the rectified voltages applied to the audio amplifier. In accordance with my invention I'connect the visual indicator to the volume control device so that the voltage applied to the visual indicator will be substantially the same at all times when the volume control is set for normal room volume. In order to insure proper response of the indicator when the volume control is turned off during tuning to prevent noise and also when the volume control is turned up to produce maximum volume, I connect between the volume control and the visual indicator a voltage divider arrangement which holds the voltage impressed on the indioator between predetermined limits and the operation of which will be more fully described below.

Referring to the drawing, the signal is received by the antenna I connected to the input of amplifier 2. The output of this amplifier is in turn coupled to a detector or rectifier by means of the transformer 3, the secondary of which with the condenser 4 is connected across the thermionic rectifier 5, the rectified output of which appears across the filtering condenser 6, resistor l and the manual volume control device comprising resistor 8 provided with the movable arm 9 for varying the setting on the resistor portion 8 of the volume control device. The rectified voltage comprises an audio frequency voltage component and a more or less constant direct Voltage component. The voltage appearing across resistor l and volume control resistor 8 is impressed for automatic volume control action in a conventional manner on the amplifier 2 through the audio frequency filter comprising resistor I0 and condenser I I, sothat only a direct current voltage, dependent on the strength of the signals is applied to the control grid of the amplifier 2 to control its amplification.

The audio frequency voltage is impressed on the audio frequency amplifier through the coupling con-denser I2 connected through the arm 0 of the volume control to the audio frequency amplifier I3, the output of which is connected in turn to an output device such as a loudspeaker I l.

The visual voltageindicating tube, which may 7 for example be of the type disclosed in Patent 2,051,189 granted August 18,1936 to Herbert M. Wagner and assigned to Radio Corporation of America and which is also connected to the output of the rectifier 5 comprises an envelope I5 containing the two section cathode I6, amplifier grid I! and anode I8 around one section of the cathode and the visual indicator comprising the fluorescent anode and control electrode I9 around the other section of the cathode. The anode E8 is connected to the control electrode iii of the visual indicating portion of the tube and determines the pattern on the target or fluorescent anode 20.

The pattern is determined as will be more fully explained below by the amount of current flowing through the resistor 2| connecting anode i8 and target 20.

In accordance with my invention the grid ll of the visual indicating tube is connected to the movable arm 9 of the manual volume control for the loudspeaker through the resistor 22, which with the condenser 23 forms an audio frequency filter so that only direct voltage is applied to grid I1.

With the circuit 'de-tuned so that very little current fiows through the output of the rectifier 5, there is very little drop in resistors l and 8 and the point of contact of the volume control arm 9 and the grid I! of the indicator tube I5 approach the potential of the cathode of the rectifier 5 and the visual indicator cathode I6 both of which are connected to ground. This permits a fiow of current to the anode I8 and hence a drop through resistor 2! so that the anode I8 and control electrode I9 become more negative with respect to the fluorescent anode 20 connected directly to the voltage supply which is constant. This causes a shadow to appear on the anode 20, the size of which is determined by the size of resistor H and values of the various elements of the circuit.

When the radio receiving circuit is tuned to resonance the rectified current flowing through resistors l and 8 will be a maximum and hence the voltage drop across the resistors l and 8 will be a maximum. Inasmuch as the cathode of the rectifier 5 and the cathode of the visual indicator tube I5 are connected to ground, both the point of contact of the arm 9 of the volume control device as well as the grid I! will be negative with respect to the cathode Hi, the negative voltage being a maximum at resonance. This reduces the fiow of current to the anode- I8 of the voltage indicator tube and hence the drop in resistor 2! so that the fluorescent anode 20 and the control electrode I9 approach the same voltage. Under these conditions the electrons reach substantially all of the fluorescent anode so that it appears as a luminous glow covering the entire surface of the fluorescent anode.

The amplitude of the voltage applied to the grid ll of the indicator and the input of the audio frequency amplifier I3 is dependent upon the position of the volume control arm 9. Since the volume control will usually be set for any particular station to obtain normal room volume, the bias voltage applied to the audio frequency amplifier as well as to the grid of the visual tuning indicator tube will be substantially the same for practically all stations.

In order to obtain silent tuning the volume control arm 9 is moved to the right so that the volume control is turned off that is grounded, and no voltage is applied to the audio frequency amplifier. Under these conditions it would be difficult to tune the set by means of the visual indicator because the grid Il then being at the same potential as the cathode it or at ground potential which does not change, there would be no indication of resonance conditions of the radio receiving circuit when tuned to an incoming signal. To permit silent tuning and at the same time obtain visual indication of tuning, the resistor 24 is connected between the grid I1 and the negative side with respect to ground of the volume control device so that a voltage divider action takes place and the voltage appearing across the volume control resistor 8 when the set is tuned to resonance is also applied across resistors 24 and 22 and hence a portion of this voltage to the grid I! of the visual indicator. On the other hand, if the volume control arm is moved to maximum volume position so that the movable arm 9 is at the negative end of the resistor 8, on strong signals the pattern on the fluorescent anode 20 of the visual tuning indicator would overlap, making it difiicult to properly tune the set visually. To prevent this overlapping a second resistor is connected between the grid I1 and the grounded side of the volume control resistor 8. Under these conditions the voltage which is applied to the grid I1 is always less than the maximum voltage applied to the audio frequency amplifier since here again a voltage divider action takes place since the voltage appearing across the resistor 8 is applied across resistors 22 and 25.

In one circuit arrangement employing my invention, the resistor 1 had a value of 10,000 ohms and the volume control resistor 8 a value of 230,000 ohms. The audio frequency filter for the automatic volume control had a resistor I0 of 2 megohms and a capacity II of .1 microfarad. The resistors 22, 24 and 25 had the values, respectively, or" 2 megohms, 6 megohms and 2 megohms, the filtering condenser 23 being of .1 microfarad capacity. The resistor 2| for thevisual indicating tube had a value of 1 megohm.

While the circuit arrangement above described is particularly adaptable for thermionic visual tuning indicator tubes of the type described, it is equally applicable to any visual voltage indicator used in combination with a circuit of the type described since in other forms of visual indicators the movable member or indicating member would be subject to the same difficulties when used with the detector circuit as in the present case that is, if adjusted for weak signals the defiection of the movable member off the scale would occur on strong signals where if adjusted for strong signals imperceptible movement of the movable member would occur on weak signals. With my invention not only is it possible to tune visually for both strong and Weak signals, but it is possible to tune silently with the volume control moved to the off position.

While I have indicated the preferred embodiment of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. A radio receiving circuit including a rectifier for rectifying alternating voltage impressed thereon, an output for said rectifier including a volume control device comprising a resistor and a movable contact arm, an amplifier coupled to the output of said rectifier through the movable contact arm of said volume control device, and a visual voltage indicating device connected through the movable contact arm of said volume control device to said rectifier.

2. A radio receiving circuit including a rectifier for rectifying alternating voltage impressed thereon, an output circuit for said rectifier including a volume control device comprising a resistor and a movable contact arm, an audio amplifier coupled to the output circuit of said rectifier through the movable contact arm of said volume control device, and a thermionic visual voltage indicating device connected through the movable contact arm of said volume control device to said rectifier.

3. A radio receiving circuit including a high frequency amplifier, a rectifier coupled to the high frequency amplifier for rectifying alternating voltage impressed thereon by said high frequency amplifier, an output circuit for said rectifier including a volume control device comprising a resistor and a movable contact arm, an audio frequency amplifier coupled to said rectifier output circuit through the movable contact arm of said volume control device, and a visual voltage indicating device connected through the movable contact arm of said volume control device to the output circuit of the said rectifier.

4. A radio receiving circuit including a rectifier for rectifying alternating voltage impressed thereon, an output for said rectifier including a volume control device comprising a resistor and a movable contact arm, an amplifier coupled to the output of said rectifier through the movable contact arm of said volume control device, a voltage divider connected to the volume control device, a second resistor connected to said movable contact arm and a visual voltage indicating device connected to said voltage divider and to the movable contact arm of said volume control device through said second resistor.

5. A radio receiving circuit including a high frequency amplifier, a rectifier coupled to the high frequency amplifier for rectifying alternating voltage impressed thereon by said high frequency amplifier, an output circuit for said rectifier including a volume control device comprising a resistor and a movable contact arm, means connected between the output circuit and said high frequency amplifier to control the gain thereof, an audio frequency amplifier coupled to said rectifier output circuit through the movable contact arm of said volume control device, and ,a visual voltage indicating device connected through the movable contact arm of said volume control device to the output circuit of said rectifier.

6. A radio receiving circuit including a rectifier for rectifying alternating voltage impressed thereon, an output circuit for said rectifier including a volume control device comprising a resistor and a movable contact arm, an audio frequency amplifier connected through the movable contact arm of said volume control device to said output circuit, a thermionic visual indicator comprising an envelope containing a thermionic cathode, a grid, an anode, a control electrode connected to said anode, and a fluorescent anode for receiving electrons from the cathode, the grid of said Visual indicator being connected to the movable contact arm of said volume control device.

7. A radio receiving circuit including a high frequency amplifier, a rectifier coupled to the high frequency amplifier to rectify alternating voltage impressed thereon by said high frequency amplifier, an output circuit for said rectifier including a volume control device comprising a resistor and a movable contact arm, an audio frequency amplifier coupled to said rectifier output circuit through the movable contact arm of said volume control device, and a thermionic visual indicator comprising an envelope containing a thermionic cathode, a grid and anode, a control electrode connected to said anode and a fluorescent anode for receiving electrons from said cathode, a second resistor connected to said movable contact arm and a connection between the grid of said thermionic visual indicator and the movable contact arm of said volume control device through said second resistor.

8. A radio receiving circuit including a rectifier for rectifying alternating voltages impressed thereon, an output circuit for said rectifier including a volume control device comprising a resistor and a movable contact arm, on audio amplifier connected to the output of said rectifier through the movable arm of said volume control device and a visual indicating device comprising an envelope containing a cathode, grid and anode, and a control electrode connected to said anode, and a fluorescent anode for receiving electrons from said cathode, a second resistor, said grid being resistor.

FRANCIS H. SHEPARD, JR. 

